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[RFI] Re: QRM from AM broadcast

To: rfi@contesting.com
Subject: [RFI] Re: QRM from AM broadcast
From: Tom Cox <tomcox@iquest.net>
Reply-to: tomcox@iquest.net
Date: Wed, 31 Dec 2003 15:39:28 -0500
List-post: <mailto:rfi@contesting.com>
Thanks to all who responded to my request for info. All replies were helpful, and I'm sure I'll get something done before long. I may make use of two or more different approaches suggested, just to reduce the QRM from varioius directions -- probably a combination of an ICE filter and one or more notch filters in several places.

Thanks again, and Happy New Year to everyone on the list.

Tom, KT9OM

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Today's Topics:

1. RE: RFI FROM nearby AM station (Patrick A. Thompson Sr.)


Date: Tue, 30 Dec 2003 20:47:08 -0500
From: "Patrick A. Thompson Sr." <wa4tukhr@comcast.net>
To: "Tom Cox" <tomcox@iquest.net>, <rfi@contesting.com>
Subject: RE: [RFI] RFI FROM nearby AM station
Message-ID: <NJEBIJHBJAHPKFLOIMGBKEDCCEAA.wa4tukhr@comcast.net>
In-Reply-To: <20031230164720.5AB55319A95@dayton.akorn.net>
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Message: 1

I live about 3 miles from a 10kw station on 990khz. I also have an IC-746.

I've got enough RF coupled into my 256ft horizontal loop, used against
ground on 160, to cause an old fashioned SWR bridge into its field strength
port read up scale with noticeable modulation!

I had broadcast overload on 160 until I started using an external matching
unit (tee match style MFJ-969). I still get interference at the 2nt harmonic
but the overload is gone and 160 is usable. In this mode I don't use the
746's internal tuner. This might be the most satisfactory solution. Maybe
you could borrow a tuning unit for evaluation.

I built, years ago, a high pass filter into a Radio Shack variable
attenuator originally made for CATV use. That helped a DX-440 (consumer
quality SWL receiver) perform well. I had video sweep gear available and was
delighted to see a nice knee at 1600khz and 28 db or so rejection.  The
design was right out of the ARRL handbook. It was not designed for transmit

If you go the homemade filter route, check your junk box for discarded audio
cassette recorders.  I found a useful adjustable coil from the bias
oscillator section.

You might also consider a pair of bypass relays for transmitting.


-----Original Message-----
From: rfi-bounces@contesting.com [mailto:rfi-bounces@contesting.com]On
Behalf Of Tom Cox
Sent: Tuesday, December 30, 2003 11:47 AM
To: rfi@contesting.com
Subject: [RFI] RFI FROM nearby AM station

Happy Holidays, Everyone!

I am looking for a practical, reasonable-cost way to keep RF from a nearby
AM broadcast station out of the receiver of my IC746 when I have a 160M
half-sloper connected to it. The station is three miles away, transmitting
1 KW on 1340 KHz.

I would like to notch the station out, if possible, so I
can listen to other BCB and LF stations on the half-sloper when I am not
using it for transmitting. If that's not possible, I would guess a
high-pass filter is my next alternative, but I will entertain all

The IC746 doesn't have a separate RX port, so I need a filter with 50-Ohm
ports that is capable of handling 100 Watts in transceive service.

I welcome your input.

Tom, KT9OM

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